Central bobbin shuttles are oscillating members which cooperate with a needle translating vertically in an alternating manner to form a lock-stitch seam.
Shuttle units of the prior art are composed of a shuttle body, also referred to herein merely as a shuttle, with a hollow semicylindrical shape, a bobbin case hinged inside the shuttle body and containing a thread bobbin and a shuttle driver guided in alternating rotation to force the shuttle body to oscillate in an alternating manner around its own axis. The shuttle body is in fact constrained to a special circular guide called a shuttle holder which allows only rotary movement thereof.
The shuttle body is provided at the top with a protruding point which grips the loop formed by the needle thread when the latter begins its upward stroke. The shuttle body, in its movement of clockwise rotation, after having gripped the loop with the special point, brings the latter to position itself in a special recess called a loop divider and, continuing in its rotation, widens the loop until it wraps around the bobbin.
At this point, under the action of the shuttle driver, the shuttle body, which has completed more than 180°, reverses its direction of rotation to return to the initial position and, at the same time, the loop, sliding on the shuttle body and the bobbin case, is closed by a special member called a take-up lever, tying to itself the bobbin thread to form the so-called lockstitch.
The shuttle units currently available on the market, with the reduction of the tension on the needle and bobbin threads necessary to prevent puckering of the fabric to be sewn, have the disadvantage of forming thread eyes in the seam and presenting uneven sewing. Vice versa, excessive tensions on the threads can lead to breaking of the thread or puckering of the fabric to be sewn.
To obtain good sewing performances it is essential, therefore, for the shuttle body to have a shape able to accompany as much as possible the movement of the loop of thread that wraps around the bobbin.
Furthermore, shuttle drivers of the new generation are made of plastic and are therefore far cheaper than shuttle drivers made of metal, but nonetheless these shuttle drivers, because of their shape, imposed by the fragility of the material and by the typical problems of plastic moulding, are larger in size and, once the shuttle driver is mounted, significantly reduce the clearance for the thread loop.
Reduction of the clearance can lead to jamming of the thread loop resulting in greater tension on the thread and a consequent unevenness of the seam.